1. The Field of the Invention
The present invention relates to flowable concrete mixtures and cured concrete obtained therefrom. More particularly, the present invention relates to concrete mixtures that have an aqueous based foam admixture and a hydration stabilizer that stabilizes the foam in the concrete.
2. Related Technology
Concrete mixtures are composite materials that are usually composed of water, cement, and aggregate. Common aggregates include sand, gravel, or crushed stone. Concrete is a well-known structural component with typical compressive strengths of about 2500 psi, when cured.
Admixtures are often added to concrete to give the concrete desired properties. Examples of suitable uses for concrete admixtures include lowering the concrete's density, improving the concrete's workability, improving the concrete's strength to weight ratio, giving the concrete insulating properties, and/or enhancing the acoustic properties of the concrete, among others. These beneficial properties are often accomplished by adding several different admixtures.
Air entraining agents are often used alone or in combination with other admixtures to give the concrete one or more of the foregoing properties. Air entraining admixtures are used to purposely trap microscopic air bubbles in the concrete. Air entrainment dramatically improves the durability of concrete exposed to moisture during cycles of freezing and thawing. In addition, entrained air greatly improves concrete's resistance to surface scaling caused by chemical deicers. Air entrainment also increases the workability of fresh concrete while eliminating or reducing segregation and bleeding.
There are several methods for entraining air in concrete. One approach is to generate bubbles or air pockets in situ. This approach can be accomplished using surfactants or other air entraining admixtures that generate bubbles as the concrete is mixed. Materials used to achieve these desired effects include non-ionic, cationic, and anionic surfactants, natural and synthetic resins, fatty acids, proteinaceous material, sulfonated hydrocarbons, and the like. This approach, while effective for entraining small amounts of air, is difficult to use for higher percentages of air because the bubble size, amount of bubbles, durability, and distribution of bubbles within the concrete are difficult to control.
Another approach for incorporating air in concrete is to add foam admixtures. When using a foam admixture, the cellular bubbles (i.e. the foam) is formed ex-situ and then mixed with the concrete. This method is advantageous because the foam is created in a controlled environment. The water, air, and foaming concentrates can be designed to produce foams having a desired size and composition.
Despite many advantages, the use of foam admixtures formed ex-situ have had limited use (e.g. in insulating, non-structural, or non-load-bearing bearing applications). Most commercially available foaming agents are not stable in cementitious media and the results of their use are often inconsistent. For example, with existing foam-concrete mixtures, the size and distribution of foam cells have been difficult to control and the cells have had a limited period of usefulness or lifetime. The foam cells have tended to agglomerate, coalesce, and recombine to give larger cells and a wide range of sizes. Long mixing times, such as those required for transportation from a concrete production facility to a construction site, have been precluded due to bubbles collapsing and air escaping from the mix. Even when additives have been used to stabilize these foams (e.g., U.S. Pat. Nos. 5,160,540 and 6,153,005, which are hereby incorporated by reference), the foam's stability and useful lifetime in the concrete prevents use in many applications, such as those requiring transportation of the concrete (e.g. ready mixed concrete).